The removal of chemical contaminants from wastewater and ground water has become an important problem in restoring ecological balance to polluted areas. It is known that some algal species are capable of absorbing heavy metals into their cell walls, thus reducing their toxic effects. Algae can also take up nutrients that may be present in overabundance, such as potassium and nitrogen, thus providing a remediating ecosystem. The system used to effect this uptake is known as algal turf. A further advantage to this technique is that the enriched algae can be harvested and used as animal feed, thus returning the nutrients to the food chain.
Algal turf can potentially be used for a variety of applications. For example, the turf can be used to replace the biological or bacteriological filters in aquaria. As mentioned above, algal turf can be used to remove nutrients and other contaminants from polluted waters. Finally, by harvesting the algal mass, various process technique can be used to produce biomass as an energy source such as methane or ethanol, as a fertilizer or as a human or an animal food additive or supplement, cosmetic or pharmaceutical.
Studies in algal turf production are known in the literature. For more than 20 years, tropical reefs have been acknowledged to be among the most productive of natural systems. For example, in Lewis, "Processes of Organic Production on Coral Reefs," pp. 305-347, 52 Biol. Rev. (1977), production values as found, for example, on p. 312 therein, indicate that coral reefs are among the highest producers in primary production values for pelagic, benthic, and terrestrial ecosystems.
Notwithstanding the values demonstrated in some earlier literature, recent efforts have demonstrated that those estimates of reef primary productivity were conservative. The mean reported value, 10.3 Gc/m.sup.2 /day should be contrasted to values ranging from 19.2 to 32.7 Gc/m.sup.2 /day in a 1980 study referring to St. Croix reefs. Such recent studies have demonstrated that algal turfs in conjunction with wave surge have been identified as the primary source of most reef productivity. The latest large-scale pilot plants in fresh water agricultural irrigation waters algal turf scrubbers or periphyton scrubbers with variable wave energies have repeatedly demonstrated production averaging 35 g/m.sup.2 /d with peaks well over 40 g/m.sup.2 /d.
Within this technology it has been known that the removal or severe reduction of wave surge motion can reduce primary productivity, subtle manipulation of sometimes very light wave energies of various patterns across the growing surface can fine tune the performance of periphyton filters or algal turf such that a desired speciation of algal turf can dominate, and thus specific forms of a particular pollutant can be more effectively removed. In some areas such as reef systems, a typical daily cycle of oxygen concentration in a reef microcosm can be greatly affected by wave surge action. Reef production is accurately measured only near oxygen saturation, since atmospheric exchange is a factor at higher or lower oxygen concentrations. When a wave generator used in such reef microcosm devices is stopped, given the same current, light, and nutrient levels, net productivity is nearly zero. The lack of an oxygen spike when the wave generator is restarted indicates that greatly reduced production is a real factor as opposed to an apparent condition because storage has not occurred.
Algal turf techniques have been disclosed in U.S. Pat. No. 4,333,263, issued to Adey, entitled "Algal Turf Scrubber," which issued Jun. 8, 1982, and the present inventor's U.S. Pat. No. 5,131,820, entitled "Low Pressure, Low Head Buoyant Piston Pump for Water Purification."
Additionally, within the reported research in this technology there is a body of literature dealing with algal techniques for waste recycling, oceanic farming, or the like. Contemporary research can be grouped in two distinct categories: those utilizing macro algae and those using planktonic algae. In the first group, macro algae reports dealing with waste recycling or the like can be found in Ryther et al., "Physical Models of Integrated Waste-Recycling Marine Polyculture Systems," Aquaculture, 5, 163-177 (1975); California Institute of Technology, Graduate School Project "Evaluating Oceanic Farming of Seaweeds As Sources of Organics and Energy, "U.S. Department of Energy, Division of Solar Technology, Contract E (04-3)-1275; and Washington State Department of Natural Resources, Project "Aquaculture of Seaweeds on Artificial Substrates," U.S. Department of Commerce, Contract R/A-12. In the case of planktonic algae, Goldman et al., "Relative Growth of Different Species of Marine Algae in Wastewater-Seawater Mixtures," Marine Biology, 28, 17-25 (1974); Karolinska Institute, "Investigation of an Integrated Aquatic System for Storing Solar Energy in Organic Material," Namnden for Energiproduktionforskning, No. 53 3065 062; and State of Hawaii Natural Energy Institute, "Energy from Algae of Bioconversion and Solid Waste," Hawaii State Government, demonstrate the status of contemporary research using that type of algae.
In either case, research to date has not utilized wave surge motion as discussed herein to enhance the exchange of metabolites between algal cells in the water medium. Also, these known research techniques have not recognized the criticality of macro algae size, vis-a-vis the shading of one cell by another. Accordingly, such techniques are not suitable for optimum biomass production, and the propensity of removing nutrients and other contaminants from polluted waters is severely limited.
Utilized in conjunction with this invention are micro algae of the major groups of benthic algae. In such algae, the use of attached algal turfs, wherein the simple algae all or most cells are photosynthetic, demands critical attention to wave surge motion. By optimizing such surge motion together with harvesting techniques, metabolite cellular-ambient water exchange is optimized, and continuous shading of one cell by an adjacent cell is prevented.
Algal turf growth can be achieved in an aqueous environment by providing a suitable vacant area in which spores may settle. The first colonizations are usually microscopic diatoms, which are then rapidly dominated by the turf species. In accordance with the present invention, the harvesting of such turfs must occur before they are overgrown in turn by the larger macroalgae or macrophytes. This keeps production rates at a high level and minimizes predation by grazing microorganisms. The rate of harvesting is dependent on light levels, temperature, water culture nutrient concentration, micronutrient concentration, and surge action. Immediate regrowth of the algal turf will occur if the vacant surface or substrate is sufficiently coarse to allow a filamentous base of the algae to remain following harvesting. Typically, such a substrate can be a plastic screen having screen grid dimensions in the range of approximately 0.5 to 5 mm, or other highly textured surfaces.
In the past, harvesting was accomplished by simply scraping the algae off the surface, but this often served incompletely to remove portions of the algae and allow these fragments and particles to be discharged into the water system, whereby the nutrients previously incorporated into plant mass or otherwise trapped were dislodged, decomposed, broken into small pieces, and flushed back into the waterway upon restart of process design flow rates. It was to improve upon the procedure of growing, harvesting, and processing the algae and other trapped particulates and organisms on a large scale (acres or more) and construction of facilities in an economical fashion, across various geological surfaces with low bearing pressures, which optimize growing conditions for the algal or paraphytic community and allow effective removal of bioassimilated or trapped pollutants after they have been taken up from the water, that the present invention was developed.